1. Field of the Invention
This invention relates to a pneumatic booster device used for reducing a pressure required to step on a brake pedal or clutch pedal of an automobile and, more particularly, to a housing of the pneumatic booster device whose outer peripheral shape is elliptical.
2. Description of the Prior Art
Considering the pneumatic booster device for use in the brake system of the automobile, its housing must be designed so that it can be disassembled and reassembled, because parts, such as diaphragm, accommodated in the housing must be checked at the time of an automobile inspection.
Typical pneumatic booster device widely used nowadays has a housing of circular outer peripheral shape which is composed of a pair of mutually coupled shells that are made by subjecting metallic plates to plastic working. Coupling of such shells is achieved, as disclosed in Japanese Patent Application Laid-Open No. 55-76743, for example, by designing in such a way that one shell is provided with a flange portion which is inserted into an opening portion of the other shell and locked in place, mating abutting portions are formed respectively on the flange portion and on the opening portion of the other shell to limit the depth of insertion of the one shell into the other shell, a plurality of pawls for locking the flange portion of the one shell are provided projectingly on the opening portion of the other shell through calking process, notches are formed in the flange portion of the one shell, each being larger in size than the corresponding pawl of the other shell and becoming opposite to that pawl as the one shell is rotated to a given position with respect to the other shell, and a stopper is incorporated to prevent relative rotation of both shells in a normal state.
On the other hand, taking account of limitations of space where the pneumatic booster device is to be accommodated, a device whose housing is made elliptical in outer peripheral shape was recently proposed and put to practical use, as disclosed in Japanese Utility Model Application Laid-Open No. 58-22369 and "Jidosha Gijutsu" Jidosha Gijutsu Kai, Vol. 37, No. 6, 1983, page 622. According to such a type having a housing of elliptical outer peripheral shape, because a coupling mechanism of both shells cannot be made identical to that of the type whose housing is circular in outer peripheral shape, flange portions are formed on the outer peripheries of opening portions of both shells, both flange portions are secured by a number of bolts, and, in relation to such securing mechanism, each shell is formed of a casting.
However, because of being formed by a casting, both shells are thick, and their flange portions are made wide for the purpose of being secured by bolts; thus, the conventional device is large in size and weighty in taking into account the relative size of the area for applying a unit difference of pressure to a power piston. Further, because a number of bolts are used, it takes much time for assembly and disassembly, thereby giving rise to high costs.